docs: def _find_literal(s, start, level, parts, exprs):
      """Roughly Python/ast.c:fstring_find_literal"""
      i = start
      parse_expr = True
  
      while i < len(s):
          ch = s[i]
  
          if ch in ('{', '}'):
              if level == 0:
                  if i + 1 < len(s) and s[i + 1] == ch:
                      i += 2
                      parse_expr = False
                      break
                  elif ch == '}':
                      raise SyntaxError("f-string: single '}' is not allowed")
              break
  
          i += 1
  
      parts.append(s[start:i])
      return i, parse_expr and i < len(s)